In a typical environment where RFID tags are used, RF signals transmitted by an RFID reader may take multiple paths to reach an RFID tag's antenna due to reflections of the RF waves from various objects in the propagation path, such as floors, ceilings, and walls. Due to constructive and destructive interference among the RF waves traveling different paths, electromagnetic standing wave patterns may be established. The standing wave patterns have periodic peaks and nulls that are located one quarter wavelength apart. An RFID tag's antenna essentially samples the RF field at its feedpoint. Consequently, if the RFID tag's antenna feedpoint is located at a null of the standing wave pattern, the tag will not receive the RFID reader's RF transmission and will not be powered up.
Diversity in antenna configurations, including spatial diversity, polarization diversity, pattern diversity, time diversity, and frequency diversity, has been explored in handheld radio systems, such as cellular phone systems, where both the transmitter and receiver are active devices. Diversity and/or an increase in signal power is used to provide better reliability in RF propagation environments where multipath fading can occur.
It should be noted that RFID tags are regulated by Gen 2 protocol standards and thus are not permitted to exploit signal processing to improve RF signal transmission reliability. Thus, there is a need for a system that overcomes the multipath fading problem, as well as providing additional benefits, for a passive RFID tag responding to an RFID reader's RF transmissions. Overall, the above examples of some related systems and associated limitations are intended to be illustrative and not exclusive. Other limitations of existing or prior systems will become apparent to those of skill in the art upon reading the following Detailed Description.